1. Field of the Invention
The present invention generally relates to recording heads in a thermal recording apparatus, and particularly relates to a thermal head suitable for superfine and halftone recording with high-accurate heating control, a driving method for a circuit for driving the head, a method for producing the head, and a recording apparatus using the head and the methods.
2. Description of Related Art
The system configuration and operation of a recording apparatus using a thermal head ar described with reference to an example shown in FIGS. 22 and 23. FIG. 22 shows the system configuration of the recording apparatus. The recording apparatus comprises an image source 24, a signal control portion 26, a thermal head 28, a micro-processing unit (MPU) 25, a mechanism control portion 27 and a mechanism portion 29. The control is as follows. Image information stored in the image source 24 is converted into a binary signal, corresponding to the recording characteristic of the head by the signal control portion 26, that includes line by line of dots, on the basis of a control signal produced from the MIP 25, and the binary signal is transferred to the thermal head 28.
On the other hand, the mechanism portion operates with the aforementioned control as follows. An ink sheet 31 and a sheet of image receiving paper 33 are nipped between a heating resistor of the thermal head 28 and a paper feeding platen roller 34. The thermal head 28 is heated in accordance with the binary signal transferred thereto, so that a one-line image is formed on the image receiving paper 33. Thereafter, take-up rollers 30 and 32 for taking up the ink sheet 31 and the receiving paper 33 rotate in synchronism with the rotation of the platen roller 34. Thus, printing image on one sheet is completed by repeating the aforementioned operation. In the case where the recorded image is a halftone picture expressing light and shade, an ink sheet, such as thermal sublimation type ink sheet or thermal paper, having halftone properties with respect to heat is used.
The halftone recording method is classified into three types as follows.
(1) A density tone stepping method in which the dots are recorded with variety in density by changing the pulse width of the binary signal corresponding to the image information.
(2) A density patterning method in which each picture element is constituted by a plurality of recording dots and the density is adjusted through changing the number of dots to be recorded in each picture element.
(3) Dither Method
One kind of the density patterning method in which N.times.M kinds of binary (for example, white and black) patterns in an N.times.M dot matrix are provided preliminarily and one of the patterns is selected correspondingly to the density.
To record a superfine, high-quality halftone image by using any of these recording methods, it is necessary to make the pitch of arrangement of the heating resistors of the thermal head 28 finer to thereby improve resolution (the number of recording dots per unit length) or to use a thermal head 28 having halftone recording characteristic or capability of changing the area (heating area) of the recording dot.
In general, the thermal head acting as described above comprises a conductor electrode formed on an insulating substrate, a plurality of heating resistors disposed thereon, and a driving circuit including driving elements for selectively supplying currents to the heating resistors to selectively heat the resistors. One of the techniques for producing such a thermal head by a thick-film process has been described in Japanese Patent Post-examin. Publication No. 55-26983. According to the technique, comb-like common electrodes and signal electrodes (called "individual electrodes" in the sense that individual signals are applied) are arranged on an insulating substrate; paste of a resistance material containing ruthenium oxide (RuO.sub.2) is provided in the form of a belt thereon by a screen printing technique. The electrodes are formed with high accuracy by applying an etching technique to a conductor layer. On the other hand, the resistor layer is formed easily by printing by using a pattern printing apparatus. Accordingly, the thick-film process has an advantage in that equipment cost is low and, accordingly, manufacturing cost for the head portion is low.
FIGS. 24 and 25 show an example of the thermal head and the circuit for driving the head.
In FIG. 24, common electrodes 2 connected to an electric power source 3 and signal electrodes 4 connected to output terminals of driving elements Q.sub.n (n=1, 2, 3 . . .) of the driving circuit 6 are formed on an insulating substrate (not shown), for example, by a vapor deposition process and an etching process; and the belt-like resistor 1 is laminated thereon by a printing process. When one output terminal, for example, Q.sub.1, of a driving element in the driving circuit 6 is turned ON, currents 7a and 7b flow from the power source 3 to a signal electrode 4 and the output terminal Q.sub.1 through the common electrodes 2 and the heating resistors 5a and 5b. At this time, the heating resistors 5a and 5b are heated to color a thermal paper at portions in contact with the heating resistors or transfer ink of an ink sheet onto receiving paper. That is, a recording dot is formed.
However, the aforementioned thermal head is designed to heat two heating resistors by one signal electrode. In short, one picture element is recorded by heating two heating resistors, so that the area of the heating portion is doubled. Accordingly, it is difficult to improve picture resolution.
To improve the resolution, a method of dividing the common electrodes into two groups and connecting the groups of common electrodes respectively to electric power sources 3a and 3b as shown in FIG. 25 has been used to double the resolution by twice. The apparatus operates as follows.
The electric power sources 3b and 3a are set to be in an electrically open state (high impedance state "Z") and a state of voltage V.sub.H, respectively. In the case where the driving elements Q.sub.1 and Q.sub.2 in the driving circuit 6 are in ON state, the current 7a flows into the heating resistor 5a but no current flows into the heating resistor 5b because the output terminals Q.sub.1 and Q.sub.2 have the same potential. However, when the output Q.sub.2 is turned OFF, the stray current 7c is one third as much as the current 7a because resistance in the current path is increased by three times compared with that of the current path for the current 7a.
In the case of binary image recording, the boundary between the binary values indicating the recording density to be printed is about half the maximum value thereof, so that the printing is not affected by the stray current.
Half the number of dots in one line are first recorded by the aforementioned operation. Then, the voltages of the electric power sources 3a and 3b are switched over so that the electric power sources 3b and 3a are set to V.sub.H and "Z", respectively. As described above, the output terminals of the driving elements Q.sub.n of the driving circuit 6 are operated corresponding to the binary image signal, to thereby record the remaining half of the dots. Thus, recording of all dots on one line is completed.
The aforementioned conventional apparatus has the following problems.
(1) In the case where resistors having a large difference in the resistance value between adjacent heating resistors, in particular, thick-film resistors, are used, it is difficult to improve resolution.
(2) Because two heating resistors disposed between the individual electrode and the common electrode are simultaneously energized and heated by the output of one driving element, it is difficult to correct the difference in heating characteristic caused by the variations in the resistance value of the heating resistors.
(3) Although it is possible to increase resolution by twice as described above, undesired currents flow through heating resistors which should be not heated. Accordingly, not only is it difficult to have high-accurate heating control but there is also a loss through a large consumption of electric power.
(4) All dots (all heating resistors) in the thermal head cannot be operated at one time. Accordingly, it is disadvantageous in recording speed compared with a thermal head capable of operating all dots at a time.